Conventionally, an optical triangulation method is used to measure a surface shape of a measurement target object since the method allows high accuracy contactless distance measurement.
Further, recently, the optical triangulation method is widely adopted in measurement of a thickness or shape of a measurement target object such as steel plate, which is conveyed at high speed.
However, according to the conventional triangulation method, sometimes the intensity or the position of the reflected light cannot be determined accurately due to change in surface reflectance, scattering property, or the like, because the triangulation method is an optical method.
Thus, high accuracy measurement has been performed in the following manner. In particular, interference is caused from an irradiating laser by a slit to form a multi laser beam, and a bright and dark pattern of fringes is detected. Then, a shape of a brightness curve at a dark part of the bright and dark pattern and at a darker part between bright parts (shape of the well portion) is approximated by a function. Subsequently, the minimum coordinate at the dark part calculated. Consequently, a distance is calculated. Such measurement is less affected by a reflectance or a scattering property of the measurement target.
However, in order to perform high accuracy measurement by a method for detecting darker parts (well portions), the shape of the well portion needs to be capable of being approximated by a function.
The reason the shape of the well portion is disordered is because of an influence of speckle noise due to a tiny irregularity on a surface of the measurement target object.